5 Potential site risks

5.1 Dioxin risk factors

• level of contamination

• extent of contamination:

  • lateral
  • vertical

• extent of any remediation

• paved or unpaved surfaces

• management practices

• potential for off-site migration

• the receiving environment, and potential for entry to food chain

• current use and probability of change.

• extent of any remediation

• paved or unpaved surfaces

• management practices

• potential for off-site migration

• the receiving environment, and potential for entry to food chain

• current use and probability of change.

The risks from dioxin contamination can be divided into off-site migration risks and on-site risks.

5.2 Off-site migration and entry into the food chain

There is potential for dioxin contamination of surface water or groundwater. Such contamination is documented for both the Waipa and Hanmer Springs sites, and the groundwater contamination (in particular) is considered to be a direct consequence of the relatively high concentration of PCP used in the preservation process and the greater mobility of the oil carrier through the ground profile.

Previous investigations have focused on PCP contamination, and the investigations have generally shown only low levels of PCP in surface water or sediment on or close to sawmill sites. On this basis, it is expected that dioxin concentrations and the accompanying ecological risk in surface water will be very low. The information collected indicates that there are only two sites - for NaPCP used as a fungicide - where PCP contamination of groundwater has been found.

As with surface water, dioxin contamination of groundwater and ecological risk is expected to be very low. Similarly, apart from the Waipa and Hanmer Springs sites, there is a very low probability of entry to the food chain. Therefore the focus of our discussion is on the potential risk to human health (using the proposed revised criteria) on the sites themselves.

5.3 Potential on-site risks

The ubiquitous nature of dioxin in the environment means that “everyone is exposed to small background levels of dioxin-like compounds when they consume food and, to a much lesser extent, when they breathe air or have skin contact with dioxin-contaminated materials” (Smith and Lopipero, 2001). Risk of exposure may arise from dioxin contamination at a site if at least one of a number of possible exposure pathways is available.

Typically the most relevant human exposure pathways are:

• ingestion of contaminated soil or dust

• inhalation of contaminated soil or dust

• dermal exposure to contaminated soil or dust

• inhalation of vapour if the contaminant is volatile

• ingestion of contaminated produce.

As already mentioned, ingestion of contaminated water is considered to be a very low-probability exposure route.

The relevant exposure pathways were the primary focus of the risk assessment undertaken for the Waipa site in 1992, which established the criteria currently used for dioxin contamination levels for various land uses in New Zealand (see Table 5.1). It should be noted that the criterion for agricultural soil is not based on the same risk assessment approach as for the other land uses, and that the risk assessment was based on the then current WHO tolerable daily intake for dioxins of 10 pg(I-TEQ) per kg bodyweight per day. In the light of more recent data, the WHO in 1998 revised the tolerable daily intake for dioxins down to the range of 1–4 pg(I-TEQ) per kg bodyweight per day. Caution is advisable when applying Table 5.1, since these interim guidelines were developed in 1992 prior to two significant influences that might affect present-day risk calculations: the use of WHO (1998) TEQ in preference to I-TEQ and the WHO revision of the tolerable daily intake.

Table 5.1 Soil dioxin acceptance criteria for various land uses/activities

Note: for the purposes of this report, equivalent guideline values calculated on

the basis of WHO 1998 TEQ are presented in Table 4.2.

The past use of PCP in the timber industry in New Zealand may have resulted in occupational exposure to dioxins for some sawmill workers. The residual dioxin levels in soils at sawmill sites (existing and former) may result in current exposure, and the risk potentially associated with this needs to be considered in three contexts:

• ongoing sawmill use (or some other industrial/commercial use)

• change in use to residential sites

• change in use to agricultural use.

We will now look at each of these in turn.

Ongoing sawmill/industrial use

The following points focus on operating sawmills but are also applicable to situations where the site use has changed to another industrial/commercial use. Current exposure for sawmill, or on-site maintenance, workers may arise from contaminated soil or dust, through ingestion or inhalation, or through dermal absorption.

1. For former small-use sites, the dioxin concentrations are below the guideline value for an unpaved industrial site.

2. For medium-use sites, the dioxin concentrations at the majority of locations are below the unpaved guideline value, but where they exceed this value they are well below the guideline value for a paved industrial site.

3. For large-use sites, 80% of the dioxin concentrations are below the unpaved industrial site guideline value and the concentrations only exceed the guideline value for a paved industrial site at two locations.

It is estimated that the majority of operating sawmill sites have paving covering the more contaminated areas. This is particularly so for the large- and medium-scale users.

Residential use

1. For small-scale users of PCP, the majority of the study results are below the residential criterion. For the sites with concentrations above the guideline value, a change in land use could increase the risk of exposure but the potential for risk is relatively low.

2. The results for medium- and large-scale PCP use are generally well above the residential criterion. If the land use were to be changed to residential without any remediation and risk reduction, there would be a significant increase in the risk of exposure.

Agricultural use

1. Even for small-scale PCP use sites, many of the results are above the criterion.

2. For the medium- and large-scale-use sites, the criterion is exceeded many-fold for all but three samples. If the land use were to be changed to agricultural without any remediation and risk reduction, there could be a significant increase in the risk of exposure.